ENGELMANN RE\ISION OF THE GENUS PINUS, ETC. 165 



both surfaces in the Pinaster section ; only P. Balfoziriana has 

 none on the back, and thus approaches Strobus in this as it does 

 in many other respects. In Strobus we find on the back no or but 

 few stomata, or sometimes a single or an interrupted line of them. 

 P. Pa?nberttana only has numerous stomata on the back, thus 

 approaching Pinaster. 



I will have to dwell somewhat extensively on the internal struc- 

 ture of the leaves, as it proves to be of the greatest impor- 

 tance lor the classification of the species. We distinguish in a 

 transverse section the thick epidermis, the chlorophyll- bearing 

 parenchyma cells, and in the centre the fibro-vascular bundle. 

 This latter is single in the terete and mostly in the quinate 

 leaves ; it is double in the broader triangular or ternate, and 

 in the semi-terete or binate leaves. This difference, however, 

 is of very little diagnostic importance, as we find occasionally 

 single or double bundles in the same species. The fibro-vascular 

 bundles always show wood cells on the upper or ventral, and bast 

 cells on the lower or dorsal side, traversed by delicate medul- 

 lary rays, usually obliquely diverging from the lower to the upper 

 «ide. The bundles are imbedded in a mass of small (medullary.?) 

 cells, free of chlorophyll, and are together with those surrounded 

 and separated from the parenchyma by a sheath of larger cells, 

 also destitute of chlorophyll. 



Within the parenchyma of the leaf a smaller or larger number 

 of longitudinal tubes or ducts are found, the resin ducts, nor- 

 mally probably two, but very often more, even as many as a 

 dozen or more. These ducts occupy a certain definite position 

 within the leaf They lie (i) close to the e^\dtYm\^, peripheral 

 ducts., in some species more on the ventral, in others more on the 

 dorsal side of the leaf; or (2) they occupy a place within the 

 parenchyma and surrrounded by it on all sides, parenchymatous 

 ducts; or (3) they lie close to the sheath which surrounds the 

 vascular bundles, internal ducts. This position of the ducts is 

 so constant, and seems to be so intimately connected with the 

 essential character of the plant, that I venture to adopt it as one 

 of the principal characters for the subdivision of the genus. I 

 must add, however, that in some few species smaller, accessory 

 ducts do sometimes occupy an abnormal position. Thus I find 

 ■occasionally in some 6Yr<9(5/, especially in P. ^^-ce/^a, where there 



